To state the obvious, plants cannot live without water - a protracted dry spell in the summer months can result in serious losses the plants most at risk. Newly planted shrubs and trees, bedding plants, shallow-rooted vegetables and climbers growing close to a house. Even deep rooted well established plants like Roses can suffer, trials have shown that growth is impaired and the flowering season is curtailed if these plants are not watered during a dry summer.
As with all garden plants, the battle against water shortage begins well prior to the dry days of summer. Incorporate adequate organic matter into the soil before planting or sowing, and ensure that the soil is completely moist to a depth of about 9 inches when planting or sowing. Mulch in late spring - you will have now done all of the preparatory work that you possibly can.
Soil with an average crop of plants loses about four and a half gallons of water per sq. yd per week in the summer and 2 gallons per week in spring and autumn. This is equivalent to 1 inch of rain water in summer and 1/2 inch in spring or autumn. If there is no rain and you have not watered the ground, this water comes from your soil's reserve and drying out occurs.
A point is reached when there is not enough water left to support healthy plant growth, and foliage starts to appear dull. Leaf rolling is soon followed by wilting and leaf fall, the final stage is death.
The solution is, obviously, to water..... but to do this properly is not as simple as it sounds. Timing, quantity and method all have to be considered. There is, on the other hand, a few general principles.
A plant should never be left until it starts to show visible signs of distress during a prolonged period of drought. Wilting means that you have waited too long, the time to water your plants is when the soil below a couple of inches depth is dry and the foliage looks dull.
Never apply a small quantity of water (less than 1 gallon per sq,yd) and then repeat the watering every few days. This constant soaking of your surface and water-starvation of the lower root zone results in rapid evaporation, surface rooting which can be damaged in hot weather, and germination of weed seeds.
Choose from overall watering and point watering. When you have a substantial area go through and lots of plants of various sizes, then overall watering must be your choice. This involves watering an area instead of restricting the watering to the root zone of every individual plant. Some people use a watering can, but you actually do will want a hose pipe if watering is not to be a prolonged chore.
The usual system is to walk slowly along the borders and around the beds with a hand-held hose fitted with a suitable nozzle. a sprinkler makes the task easier and is necessary for all but the tiniest of lawns.
The better approach to watering vegetables and shrubs are the sprinkler hoses and seep hoses, but they can be expensive. Point watering is used where there is a limited amount of large plants to deal with. The methods used are all designed to limit the water to the immediate zone covered by the roots of every plant.
Water thoroughly when you have decided to water. If you are using a watering can, remove the rose. Hold the spout near the base of the plant and water slowly. If you use a sprinkler water in the evening - never in hot sunshine. With overall watering apply 2 - 4 gallons per sq. yd, using the higher amount during midsummer in sandy soils and with high risk plants.
With point watering use 1 - 4 gallons per plant, depending on the size of shrub or tree. Repeat the watering if there is no rain. There is no easy way to work out the right time to do this repeat watering. Dig down with a trowel and examine the soil at 3 - 4 inches below the surface. If it's dry, then water. As a general rule watering will probably be required about every 7 days during a period of drought.